More than a million customers in Puerto Rico remained without electricity on Thursday after a fire at a main power plant caused the biggest blackout so far this year across the U.S. territory, forcing it to cancel classes and shutter government offices.

The major power outage served as a reminder to residents that essential work to modernize island’s antiquated and unreliable electric grid, which was also decimated by Hurricane Maria in 2017, has not yet begun.

The blackout also left some 160,000 customers without water and snarled traffic across the island of 3.2 million people, where the roar of generators and smell of diesel filled the air. Those who could not afford generators and have medical conditions such as diabetes, which depends on refrigerated insulin, worried about how much longer they’d be without power.

Luma, the company that took over transmission and distribution from Puerto Rico’s Electric Power Authority last year, said the blackout could have been caused by a circuit-breaker failure Wednesday at the Costa Sur generation plant — one of four main plants on the island.

Image: PUERTO RICO-ENERGY-POWER OUTAGEPedestrians walk on a dark street Wednesday in San Juan, Puerto Rico, after a major power outage hit the island.Ricardo Arduengo / AFP – Getty Images

“The system is being restored little by little,” said Kevin Acevedo, a vice president of Luma, adding that the company is trying to complete the work within 24 hours. “The people of Puerto Rico have to understand that it’s a system with a lot of years. Bringing back Puerto Rico’s system is a delicate and complicated process.”

Luma said the exact cause of the interruption is unknown.

“It’s going to require an exhaustive investigation,” Acevedo said, adding that that the equipment whose failure sparked the fire had been properly maintained.

Officials said at least three generation units were back online by Thursday, with crews working to restore more.

The outage occurred two months before the Atlantic hurricane season starts, worrying many about the condition of Puerto Rico’s electrical grid.

“Yes, the system is fragile, no one is denying that, but we’re prepared,” Acevedo said.

Police officers were stationed at main intersections to help direct traffic on Thursday while health officials checked in at hospitals to ensure generators were still running.

Image: PUERTO RICO-ENERGY-POWER OUTAGEA man holds up the light of his mobile phone so that another can read from a notebook in San Juan.Ricardo Arduengo / AFP – Getty Images

The outage further enraged Puerto Ricans already frustrated with an electricity system razed by Hurricane Maria in 2017. Emergency repairs were made at the time, but reconstruction efforts have not yet started, and power company officials blame aging, ill-maintained infrastructure for the ongoing outages.

A series of strong earthquakes that struck southern Puerto Rico where the Costa Sur plant is located also had damaged it.

The Electric Power Authority also is trying to restructure $9 billion worth of public debt to emerge from a lengthy bankruptcy. The company has struggled for decades with corruption, mismanagement and a lack of maintenance.

In June last year, a large fire at a substation in the capital of San Juan left hundreds of thousands without power. Another fire at a power plant in September 2016 sparked an island-wide blackout.

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